In thin film metallurgy the detrimental effects of diffusion of atoms from one film layer to another have long been realized. Any interdiffusion of atoms from another layer generally means a change in characteristics with time. Some changes in characteristics may be as simple as direct variation of a single parameter, such as resistivity, adhesion, ductility and solderability. In more complex applications, such as semiconductor devices where atomic scale impurity levels are involved, unwanted atoms appearing by diffusion can cause catastrophic device failure.
Ordinarily, diffusion is considered to be a relatively slow and predictable bulk effect. With thin films, however, the effect proceeds much faster than would be expected with merely a bulk effect. Short distances, grain boundaries and defects are thought to be involved.
The diffusion barrier structure is usually a single layer positioned in a series of layers that operates to control, prevent or retard the travel, within the film layers, of atoms from one location to another where the presence of those atoms is detrimental.
Other barrier structure layers may be inserted in a thin film stack to prevent or produce compound formation between potentially interacting layers.
In semiconductor technology, the diffusion effect and compound formation effect is aggravated not only by the sensitivity of the devices but also by the thermal stresses in the process windows in manufacture. In semiconductor technology, a process window is the temperature level and time at that level allotted to a step in the fabrication of the device or array of devices, such as an integrated circuit. In making such devices or arrays, there may be many process window temperature excursions that can have a cumulative effect.
In the semiconductor art, there are functions needed in contacts either to facilitate manufacturing or performance and a particular metal that can impart the needed function is introduced into a thin film stack for that purpose. There may be many layers, each of a different metal, for a different purpose. As examples, copper (Cu) or aluminum (Al) for conductivity, gold (Au) for corrosion resistance and solderability, and chromium (Cr) for adhesion. As the art has developed, barrier technology has evolved into single layer structures. A thin film diffusion barrier structure of nickel (Ni) for retarding diffusion of atoms between Cu generally present for conductivity and Au generally present for corrosion resistance and solderability has become extensively used. One example of such a structure is in U.S. Pat. No. 4,016,050.
Another type of barrier structure is shown in U.S. Pat. No. 4,816,424 using titanium nitride (TiN)-tungsten (W) to prevent interdiffusion of Al and silicon (Si) where Al is used as a conductor.
Still another barrier structure is shown in U.S. Pat. No. 4,478,881 where in fabrication a layer of W is placed between an Al conductor and a Ni contacting layer on a Si semiconductor device which after processing results in a single barrier layer of W that prevents a chemical reaction between Al and a NiSi contacting compound.